The DNA repeat sequence coding for the rRNA of Saccharomyces cerevisiae has been cloned in three different lambda gt cloning vectors (EcoRi, HindIII and SstI). The physical position of the regions coding for the 4 RNAs (26S, 15S, 5.8S and 5S) have been mapped by two electron microscopy methods. The two larger rRNAs will form stable R-loops which are readily mapped in the electron microscope by the basic protein film technique using formamide as the denaturant. The two small rRNAs as well as the two large rRNAs have been mapped by first hybridizing the RNAs to single standard DNA in a heteroduplex then covering the remaining single stranded DNA with E. coli DNA unwinding protein followed by antibody to this protein. The proteins are cross linked and mounted for electron microscopy by the basic protein film technique. BIBLIOGRAPHIC REFERENCES: Struhl, K., Cameron, J.R. and Davis, R.W.: Functional Genetic Expression of Eukaryotic DNA in E. coli. Proc. Nat. Acad. Sci. 73, 1471-1475 (1976). Kramer, R.A., Cameron, J.R. and Davis, R.W., Isolation of Bacteriophage Lambda Containing Yeast Ribosomal RNA Genes: Screening by in situ RNA Hybridization to Plaques, Cell 8, 227-232 (1976).